Lewy body dementia is common. It presents either with cognitive impairment and neuropsychiatric disturbance followed by parkinsonism or as dementia complicating established Parkinson's disease. It is unusual both in its pathological features and in its clinical manifestations. Although both overlap to some extent with those of Alzheimer's disease, Lewy body dementia is at least potentially recognizable during life. Some of its manifestations can be ameliorated by established methods, and it has pathological and neurochemical features which offer some hope for the development of useful palliative therapy. Major progress towards effective treatment is, however, likely to depend upon an improved understanding of the molecular mechanisms underlying its aetiology.